Foamed objects comprising a plurality of coalesced distinguishable extended strands of polymers (strand foams) have been previously disclosed, for example, in U.S. Pat. Nos. 3,573,152; 3,467,570; 3,723,586; 4,192,839; 3,954,365; 3,993,721; 5,124,096; 5,405,883; 5,288,740; 5,124,097; 5,110,841; 5,109,029 and 4,801,484. Further documents describing strand foams are WO 92/16393, EP-A-0 279 668, JP 60-015114-A, JP 53-1262 and JPH6-263909. The foamed objects are prepared by extruding a foamable thermoplastic material through a multi-orifice die plate, whereby the individual foamable elements of the strand are formed, expanded and coalesced upon emerging from the die orifices. The strands can have a circular cross-section, however, the production of strands using multi-orifice dies in which the orifices can be slots, squares, holes or specials shapes has been described. The advantages of providing the multistrand product are specified to be superior strength in the plane transverse to the direction of extrusion, extrusion to a predetermined shape without any need to trim, low density product having distinguishable coalesced cellular strands, and ease of change of shape by varying design of the extrusion die.
Many of the processes described above pertain to the production of strand foams on the basis of polyethylene, polypropylene or polyvinylchloride. Although these strand foams are excellently suited for a multitude of applications, there is a great need to provide strand foams based on other polymers, for example, polystyrene.
U.S. Pat. No. 3,993,721 discloses a process and extrusion die for preparing foam articles of thermoplastic resin, for example, polystyrene, having a hard and a smooth surface and resembling natural wood. The polymer/blowing agent mixture is extruded through a tiered die plate having a peripheral portion and a protruding interior portion. Each of the portions is provided with a plurality of apertures, the aperture density of the peripheral portion being greater than that of the interior portion. A cooling frame mounted adjacent the resin discharge plate forms a restrictive contact zone in which foamable strands are extruded and coalesced thereafter. A disadvantage of this process is that only foams of a relatively high density, for example, 200 kg/m.sup.3 and more, can be manufactured. Further, this process is restricted to hydrocarbon blowing agents and produces foams exhibiting poor insulation values and dimensional stabilities.
JP 60-015114-A discloses a process for producing a foamed product comprising bundling and binding extruded and foamed-free thermoplastic styrene resin rods so that at least the rods arranged at the outermost layer comprise partly or fully foamed thermoplastic polystyrene resin containing at least 5 weight percent elastomer and the density of the rods is reduced stepwise from the outermost layer to the inner layers. The product has a high impact resistance due to the outermost layer and a low bending strength with inner layers. The foam density varies from 300 to 600 kg/m.sup.3. A drawback of this process is that it is restricted to the production of foams having a high density. Moreover, unsatisfactory insulation values and dimensional stabilities are achieved.
U.S. Pat. No. 3,573,152 discloses a process for producing strand foams of polystyrene and polyethylene. The foamable materials are extruded through a multi-orifice die. The expandable strands are fused or coalesced into an integral shape. Blowing agent is used in an amount of 5 to 50 weight percent, wherein a cellular foam of a density from 16 to 64 kg/m.sup.3 is obtained. For polystyrene, hydrocarbons are disclosed as blowing agents. A disadvantage of this process is that the products thereof often do not possess sufficient dimensional stability and insulation values.
U.S. Pat. No. 3,467,570 discloses extruded foam thermoplastic resin boards having a cross-section at right angles to its length, consisting of a network of the extruded foamed material having enclosed within each of the meshes of the network, a strand of the extruded foamed material, such that the board as a whole has a substantially uniform cross-section. The resin, for example, polystyrene, is produced by extruding foamable polystyrene through a die having a plurality of channels in the form of a network of square or triangular meshes and each mesh has a central passage. The density of the resulting product is 14.4 to 32 kg/m.sup.3. The blowing agent is a hydrocarbon, for example, butane or isobutene. A drawback of the thus obtained foams is that their insulation values and dimensional stability are often unsatisfactory.
U.S. Pat. No. 3,723,586 is a further development of the above-mentioned U.S. Pat. No. 3,467,570. A process of extruding foamable polystyrene through a die having a plurality of channels in the form of a network of meshes is disclosed, wherein each mesh has a central passage with extended grooves to four corners of the mesh. The slits are arranged in a symmetrical network of square meshes, with extended grooves from the central passage to four corners of the surrounding mesh. As blowing agent hydrocarbons such as butane or isobutene are used. The resulting product has a density of 14.4 to 24 kg/m.sup.3. This process also generates foams exhibiting no satisfactory properties as regards to insulation value and dimensional stability.
U.S. Pat. No. 4,192,839 discloses a process for producing an expanded article of a crystalline thermoplastic resin by extruding and expanding a foamable resin mixture through a die having a plurality of apertures therein and a frame for expansion. The cross-sectional area occupied by the separate resin mixture streams at the entrance of the extrusion area is from 5 to 30 percent of the total cross-sectional area at the exit of the extrusion area. After leaving the extrusion area, the streams are passed directly into a confined zone defined by the frame and maintained at a temperature at least 30.degree. C. lower than the temperature of the resin streams prior to the exiting. This forms a plurality of soft expanded resin strands. By simultaneously removing gases generated in the course of extrusion and expansion, the formation of voids between strands is avoided. Apart from the fact that the removal of gases is very laborious, no products having high insulation values and sufficient dimensional stability are obtained.
EP-A-0 279 668 describes closed-cell foams, particularly suitable for use in packaging applications, comprising a plurality of coalesced parallel strands or profiles of thermoplastic resin, for example, an alkenyl aromatic thermoplastic synthetic resin, such as, polystyrene. The volatile blowing agents are those conventionally known for alkenyl aromatic thermoplastic resin foams. Although according to the method of EP-A-0 279 668 products of high quality are obtained, there is the problem that often individual strands do not sufficiently adhere to one another or that voids remain between individual strands.
U.S. Pat. No. 5,288,740 discloses a process for making closed-cell, alkenyl aromatic polymer foam packing bodies. An alkenyl aromatic polymer material is heated to form a melt into which a blowing agent is incorporated at an elevated pressure to form a foamable gel. The foamable gel is extruded through a die to form a foam strand. The foam strand is pelletized to form a plurality of foam bodies. These bodies are further expanded by exposing them one or more times to heated air or resin streams. Among others, carbon dioxide and water are mentioned as blowing agents. The described process, however, has the disadvantage that coalescing of the expanded pellets can only be obtained by melting the pellet surface. This, however, results in zones of elevated foam density in the area of the interfaces between individual pellets and thus to a deterioration of the insulation values. A further disadvantage of this process lies in that no continuous foam structures are obtained, thus resulting in a structure with interfaces not only parallel to the direction of extrusion, but also perpendicular to the extruded direction. Such a structure has inherently lower bond strength and poor resistance to water permeation due to the high number of interfaces per unit volume.
Thus, it would be desirable to provide a strand foam comprising a polystyrene-based resin having excellent insulating and dimensional stability properties at a low density. It would also be desirable to provide a method for producing a polystyrene-based strand foam which is simple, inexpensive and environmentally acceptable and results in products having good physical properties.